Lithium is the lightest and most electropositive element, making it well-suited for applications that require high energy density. As a result, lithium-ion (Li-ion, or Li+) batteries have become the most common battery employed in a large variety of portable electronic devices. Success in the electronic market has promoted their use in hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs). These applications as well the ever increasing demand for more powerful portable electronic devices has prompted the need for batteries that can consistently maintain large charge and large discharge current densities. Safety is also becoming an important factor in the design of new Li-ion batteries, especially for transportation applications. Currently, most Li-ion batteries include a porous separator which is soaked in a non-aqueous carbonate based electrolyte containing LiPF6. Because of this electrolyte, these batteries are prone to catching fire and can expose the surrounding environment to toxic vapors, as discussed by J. Li et al. in Journal of Power Sources, 196, page 2452 (2011).
This has led some researchers to investigate solid-state Li-ion batteries as a safe alternative to conventional non-aqueous electrolyte based batteries. However, there are two principal issues that must be addressed before solid-state Li-ion batteries may become widely accepted for commercial applications. The first is the slow diffusion of Li-ions into the anode and the cathode, as well as slow diffusion within the solid-state electrolyte separating the two electrodes, which causes the charge/discharge rates of such batteries to be inferior to conventional Li-ion batteries. The second is a consequence of the first, in that solid-state Li-ion batteries generally include thin layers to compensate for the slow solid-state diffusion, but at the expense of energy density. The diminished energy density has limited the application of solid-state batteries to those which do not require high energy density such as: autonomous wireless sensors, powered cards, active RFID/RTLS, medical devices, and memory backup power.
While reducing the dimensions of the electrode materials improves the rate performance compared to bulk materials, reducing the distance between the cathode and anode cell structures has not been straightforward, and Li-ions are required to travel large distances between macroscopically separated electrodes. Planar geometries of batteries resulting from commonly used line-of-sight fabrication techniques significantly reduces the energy density that can be obtained.